Light-polarizing image and methods for forming the same



prii 20, 1948.. E. H. LAND ET AL 2,440,106

LIGHT-POLARIZING IMAGE AND METHODS FOR FORMING THE SAME Filed Apfil 261946 FIG. 3 FIG. 4

l VENTORS M,

Patented Apr. 20, 1948 LIGHT-POLARIZING IMAGE AND METHODS FOR FORMINGTHE SAIME Edwin H. Land, Cambridge, Joseph Mahler,

Brookline, and Mark Hyman, Jr., Cambridge,- Mass., assignors to PolaroidCorporation, Cambridge, Mass., a corporation of Delaware ApplicationApril-26, 1946, Serial No. 665.084

9 Claims.

This invention relates to novel methods for forming light-polarizingmaterials and particularly has reference to forming light-polarizingimages in transparent, hydrophilic, molecularly oriented, linearpolymeric plastics.

This application is in part a continuation of our copending applicationSerial No. 434,568, filed March 13, 1942, for Light polarizers andmethods and materials for forming the same.

Important objects of the invention are to provide methods for forminglight-polarizers and light-polarizing images by predeterminedlyincorporating an oxidizing compound or agent in a linear polymericplastic element or sheet and treating the sheet with a material or agentwhich is oxidized in the presence of the oxidizing compound to produce adichroic stain in any portion of the sheet containing the oxidizingagent, the molecules of the sheet in any portion thereof containing theoxidizing agent being substantially oriented at some stage of themethod; and to provide methods of the foregoing character wherein theoxidizable material is an iodide or a mixture of iodides from whichiodine is released by an oxidation reaction with the oxidizing agent,and wherein the oxidizing reaction is brought about in the presence of asolvent for the oxidizing agent as for example an acid and particularlyby reacting the oxidizing agent with an acid solution of an iodide ormixture of iodides.

Other objects of the invention are to provide processes of the characterdescribed which are particularly adapted for the reproduction fromphotographic negatives of light-polarizing or dichroic images inmolecularly oriented plastic materials and particularly for theformationof dichroic images in such materials and their combination assuperposed rightand left-eye stereoscopic images. g

Still further and yet more specific objects are to provide a new processfor forming light-polarizing material through the employment oflightsensitive materials, particularly to utilize the direct action oflight on plastics which are capable of having their molecules orientedand which have been rendered light-sensitive as a step in the formationof light-polarizing images therein; and to provide a process whereinsuitable sheet plastic is rendered light-sensitive, differentiallyexposed to light and treated with an agent adapted to react. with theexposed material to produce a dichroic stain such as iodine in thesheet, at least the portions of said sheet penetrated by the dichroicstain being molecularly oriented at some stage of the process; as wellas to provide a (cits-6) process wherein the differential exposure ofthe light-sensitized plastic takes place through a photographic or othernegative and the resulting exposed portions are developed to a dichroicstain which, will constitute a, positive reproduction of the negative. 1j I Yet other more specific objects of the invention are to provide aprocess for forming light-polarizing material by introducing anoxidizing compound or agent into a linear polymeric plastic inpredetermined portions of the plastic whereby to provide an image formedof the oxidizing agent, and then utilizing the oxidizing agent to form adichroic stain in the portions of the plastic in which it isincorporated, at least thoseportions of the plastic containing thedichroic stain having their molecules substantially oriented at somestage of the process; as well as to provide a process for thereproduction of a photographic negative image as a light polarizingpositive image by subjecting a suitable plastic, sheet to differentialimbibition of a solution of an oxidizing compound or agent to form apositive image in the sheet in terms of the oiddizing'a'gent and thentreating the sheet witha material oxidized by said oxidizing agent toprovidea dichroic stain, in the portions of said sheet containing saidoxidizing compound, and especially to carryout such practices byproviding a positive image in the form of a wash-"off gelatin relief,saturating the relief with an oxidizing-agent and. printing on theplastic sheet with the treated wash-01f relief whereby to provide animage in the sheet comprising the oxidizing agent.

Other objects and advantages will in part appear and in part be pointedout in the course of the following descriptions ofv the invention, whichare given as non-limiting examples in connection with the accompanyingdrawings, in

which: v Fig. 1 is a diagrammatieview illustrating a convenientarrangement for carrying out the process of the invention whereinlight-polarizing images are formed, in lightesensitive plastic materialand shows the exposure'of a sheetlof a suitably sensitized plasticthrough. a" photographic negative;

Fig. 2 is a diagrammatic view representing a step in ,the process of theinvention'utilizing a wash-off relief inf the formation of alight-polarizingima'ge;

' Fig. 3 is a diagrammatic perspctiveview illustrating an image-carryingelementhaving a lightpolarizing image formed'ther'ein in accordance witheither of the practices described in connection'with Fig. 1 0 1 2; and

in transparent, hydrophilic, linear polymerid'" 4 conveniently byimbibing the sheet in a solution containing the desired light-sensitiveagent. Among the materials found suitable for this purpose, thepreferred materials comprise dichro- 5 mates, such for exampleasarnmoniuln dichromate; potassium dichroinateFand sodium, dichromate;with "ammonium "dichr'omate "being preferred for the reason that it isthe most soluble. The just-described dichromates may be used plastics.All of these ractidsigener icauyinvolve" 1o ly 1, a x fi of two or moreof them may the introduction or incorporation of lii'bxi'dizing compoundor agent within cially sheet material, and. th oxidizer to the end ofstainingthe'plast dichroic staii'l. v

A preferred process and one which is admirably suited for explaining theinvefitibii'andespecially the fundamentals thereof is illustrated in connection with Fig. 1 and comprises three main steps-treatment of a sheetof the desired ma- 20 trial with; a suitableflightfsensitiv. material,xposu'ie of the sensitized sheet to'li and the foimatizinpr sh iniagej[the s et comprising-an oxidizing agent, renew d by'ltr at l t of thesheet with an agent, which when oxidized; pro:

duees a dichroicstain in the exposed areas of tlfesheet. TInfidditidfitd: steps, it 11 laybe esirable ii i senje 'casesto add afourth step 60m: pri'sihg'tratm'"enlibf'jfiif staine sheet mantensififer barn? Ptiitiemai'noveaynesin'nr fact so r esmayconvenientlyflief'abinplis edfbyfsoft en 50 n' it e seti s f tex i ewa tr f heat; andtlien stretching dexte until"suitableorientation of mem eculesfhas been obtained, In thecase of polyvi 'yl'al'cohol,

diti'oii for the fon s non 6r diehroieimag'es-tnere; inbyextending't'o'fro awe-to four arrive-times J gthI"v swear specific"examples and formulae, relate ing to thevarious practices of theinventioiijiwill 'if' By wayyof, lllustration the invention ill'b asheaf i QQ 15in .solve be employe'cl as the light-sensitive agent.Dichromates are particularly useful as light-sensitive agents incarryingout the present invention since thesmaterials are good oxidizers.

if r; the light-sensitive agent may be simply watei, but this is notwholly satisfactory 'foi' the rea-soh that it tends to penetrate thesheet too rapidly and thus tends to soften it It is preferable,therefore, to add to the solution a liquid which is miscible therewithbut is agnon 'solvent jfof the plastic as" well" "as non.- e'abtiv th. te othe mie'a s t r e 'ixa d hi ni-a j g m m enfl lQWP PJ mi Oi i' -ithe"sheet"and thusjaid infniaintainingits stiff ness. Suitableexamplesdncilude acetone; niejth 1 1 and l h l i fi n? lume h r used=inay vary anywhre up to approximately 99% of-the totai voluine'bfsolventpresent.-

The proportion of sensitizing" agent? to solvent 7 a widerange from=aswas 1 to 2 1 'to-as-- high as 40% "based on the-"v0 urn'e rovided thevar-iatibn is compensated for by va ation *ofthe-t-i-i'rieof'trea-trnent approxi mately inversely 7 to the pro-portion ofsensitizing agent. Saidtime f treatment may-vary 'frm a fewseamlesupward but {should-preferably continue' erily long enough'tb-pfermit 'thesolution to---'pen etrat e'- the-surface of the sheet"uniformly for a short" distaiice; and in no case=sh0uldY-th sheet beallowed to -soak-so-long that it softens unduly. It has-end been rounddesirable to add a-wtting agem suehpror eXainple,-as soap-"or di'hiifflsoliillin slllffisllccilflt in order bdth -t O accelerate the sensitizintreatment andt'o pro mote -uniform-sensitizationi Asatisfactorysensitizing 'bath has the iioil l l'llla immediately given 2Formula Water cc" Acetone 1 cc; 25 Ammoniumdichromatei; g nz; '1'6Wetting "agent; gm; 1

fshouldbethroug'h'the negative-*to berep Diluted,

l Fl fl wh e n e "e fi h i re r se ted: mounted wide a "photographicne'gative-Fl'zfand sensitized, as pointed out above. If, for example, anR2 Photofiood bulb mounted approximately six inches from the surface ofsheet in be used as the light source, a sheet sensitized as describedabove for two to three minutes should be suiliciently exposed in fromforty to sixty seconds.

It should be noted that during the exposure, and particularly in thecase of exposure for a long period, it may be advisable to shield sheetl from the infra red rays emanating from the light source, as by meansof a suitable filter I5. If it is desired to print by means of theultra-violet rays emanating from source it, element l5 should alsoinclude filtering material for visible light, and the exposure timeshould be increased to about five minutes, owing to the uniformabsorption of the ultra-violet light by negative l2.

The effect of the exposureof the sensitized sheet of the invention tolight is to harden the surface of the sheet approximately proportionallyto the degree of exposure. As will be seen from Fig. 1, th degree ofexposure varies according to the variations of density in negativel2, 1. e.,

' more light is transmitted by the lighter areas of the negative than bythe darker areas. The result is that the surface of sheet ii! ishardened to a varying degree corresponding inversely to the density ofnegative [2. In other words, where the negative is lightest, sheet If]is most exposed and most hardened, and where the negative is darkest,sheet It is relatively unexposed and remains relatively soft. Thus thedichromat in the soft or relatively unexposed portions of the sheet maybe readily washed out of the sheet whereas the dichromate in thehardened or heavily exposed portions of the sheet is removed withdimculty. When the sheet is washed, fol-' lowing exposure, the efiectproduced is to leave an image therein in terms of an oxidizing agent,that is in terms of the dichromate or mixture of dichromates used forsensitizing purposes.

As intimated, the next step in this embodiment of the process is toeliminate unexposed lightsensitive material from sheet ill. This may bedone in the present example by a simple washing with Water to remove thedichromate from the unhardened areas of the sheet, and the time ofWashing varies more or less according to the degree of sensitizing andthe time of exposure, the important factors being that all unexposeddichromate should be removed but that the washing should not continue solong that the highlights of the picture are washed out or the exposeddichromate aifected. It is for this reason that the sensitizingtreatment is preferably confined to the surface of the sheet; theunexposed dichromate may then be removed inas short a time as possible.For a sheet sensitized and exposed as described above, the washingshould be completed in approximately two minutes, but it must beemphasized that this time is by no means critical and may be variedconsiderably without appreciably altering the result.

The next step in the process is to treat the exposed and washed sheetwith an agent or com-- position in solution form which is somewhatanalogous in its action to a photographic developer in that it reactswith the exposed material in 6 solution. The dichroic stain thus formeds'ub stantially replaces the oxidizing agent, that is to say, portionsof the sheet in which an oxidizing agent was present prior to theoxidation treatment just described contain substantially only a dichroicstain following such treatment.

The oxidation treatment is readily carried out with a material which isoxidized by the oxidizing agent in the sheet to give a dichroic stainand which is dissolved in a solvent material mutual to said oxidizingagent. An iodide or mixture of several iodides provides an oxidizablematerial of the character desired and when the same is reacted with theoxidizing agent in the sheet in the presence of a solvent also common tothe oxidizing agent, for example an acid, iodine is released from theiodide or mixture of iodides to form a dichroic stain, the releasediodine replacing the oxidizing agent. The iodide or iodides should bewater soluble and are preferably contain'ed in a solution which isrendered acidic.

These characteristics in the developer may conveniently be securedeither by using an alkali iodide, particularly ammonium iodide or analkali metal iodide such, for example, as potassium iodide or sodiumiodide, and adding one or more acids to the solution, preferred acidsbeing hydrochloric and hydrobromic acids. Alternatively, other acidssuch, for example, as sulphuric, nitric, phosphoric, or acetic acid maybe used. Other iodides may be employed, for example, ferrous iodide orcerous iodide, or an iodide solution which is naturally acidic such, forexample, as

hydrio-dic acid, may be employed.

It has already been pointed out that the result of the exposure stepillustrated in Fig. 1 is to harden the surface of the sensitized sheetdifl'erentially, depending upon the optical density of the photographicnegative at any given point. It follows, therefore, that after theexposed sheet has been Washed, the amount of dichromate remainingtherein will similarly vary differentially over its entire area. Thenwhen the sheet is subjected to the oxidation treatment, the iodine stainabsorbed in any particular area will depend upon the amount ofdichromate present in that area. That is to say, the stain will berelatively intense in the areas where there is considerable dichromate,namely the areas corresponding to light areas in the original negative.Conversely, there will be relatively little dichromate in the areascorresponding to the dark areas in the negative, and the amount of stainabsorbed there- In other in will therefore be substantially less. Words,the final product of the oxidation step of the operation will be apositive reproduction in light-polarizing areas or the original negativeimage represented by element l 2 in Fig. 1,

As is the case with the other agents and steps of this invention, thecomposition of the developing solution which contains the oxidizablematerial may vary to a considerable extent, depending both on theresults desired and on the way in which the preceding steps were carriedout. For example, if the sheet was sensitized in a concentrated solutionand exposed for a relatively long period, a concentrated developingsolution may be used, but if the sheet was sensitized in a dilutesolution and exposed for a short time, the developing solution and timeof treatment therewith must be correspondingly modified. Similarly, theparticular acid used affects the time of the oxidation reaction, use ofa strong acid like hydrochloric requiring a shorter treating time thanone containing a tive 12 produced by conventional photographic methods.Conversely, if analyzer 30 Were rotated through ninety degrees withrespect to print 29, it would then act to absorb the remainder of thecomponent differentially absorbed by the print, with the result that theimage would disappear and the combined elements would appearsubstantially uniformly transparent.

The process of the invention is well adapted to the production ofdichroic stereoscopic images. For example, the image shown in Figs. 3and 4 may be assumed to be one image of a stereoscopic pair. In thatevent the other image should be reproduced on a plastic sheet having itsmolecules oriented substantially at right angles to the direction ofmolecular orientation in sheet 20. The two sheets may then besuperimposed with the images thereon in relative stereoscopic registry,and the resulting composite image will have a three-dimensional effectwhen viewed through proper polarizing viewing means, in accordance, forexample, with Patent No. 2,203,687.

The terms dichroisrn and dichroic as used herein and in the claims areintended to refer to the property of polarizing transmitted light bydifferential absorption according to vibration direction. It will benoted that in its simplest form the process of the invention may bepracticed to produce a uniform dichroic light-polarizing sheet. This maybe done, for example, by sensitizing a sheet of the desired molecularlyoriented plastic, exposing the sheet uniformly to light, and thentreating as described above to produce a uniform dichroic stain therein.It is to be understood that this and similar modifications of theprocess of the invention are to be construed as coming within the scopethereof.

It should be pointed out that it is not atall essential to the practiceof this invention that the various steps of the preferred processthereof be carried 'out continuously. For example, the sensitizingtreatment may take place at an entirely different time from theremainder of the process. It should also be noted that these sheets arerelatively low in light sensitivity, and that only moderate care need betaken to protect them from exposure to extraneous light while carryingout the other steps of the process of the invention.

The foregoing practices involving the use of a light-sensitive oxidizingagent provide a relatively simple process by which dichroic images maybe easily and readily formed. However, it is to be kept in mind that theinvention has for a concept the use of any oxidizing agent which willrelease a dichroic stain from a solution of a suitable oxidizablematerial and the incorporation of the oxidizing agent in predeterminedportions of a suitable plastic by practices other than those describedas depending upon the ability of an exposed light-sensitive oxidizer tosubstantially remain in the plastic while the unexposed oxidizer isremoved by washing. Thus, the inventive concept comprehends in additionto oxidizing agents which are sensitive to light, the use of relativelylight-insensitive oxidizing agents incorporated or introduced into asheet of a suitable plastic. Many oxidizing agents in addition to thedichromates and including inorganic and organic materials are known andall are included within the scope of this invention. As a fewillustrative examples of suitable inorganic materials of this character,mention may be made of metallic salts such as ferric salts, for exampleferric chloride,

and cupric salts, for example cupric sulfate, cupric chloride and cupricnitrate, as well'as sodium salts of oxidizing. acids such as sodiumperchlorate and sodium perborate. Examples of organic oxidizing agentssuitable for use with this concept of the invention may be found inpbenzoquinone and otherquinones. The oxidizing agents disclosed may beemployed singly or a mixture of several of the same may be used.

Preferably the oxidizing agent should be water soluble and all of thosejust named are of this character. An oxidizing agent may bepredeterminedly introduced into a suitable plastic sheet by imbibitionfrom a water solution. On treatment of a sheet in which one or moreoxidizing agents have been imbibed with any of the treatin or developingsolutions heretofore described in connection with the preferredpractice, a dichroic stain in the form'of iodine will replace theoxidizing agent.

In dichroic image'formation, a water solution of an oxidizing materialor mixture of such materials preferentially imbibed into the sheet fromasaturated wash-off gelatin relief to provide a desired image comprisingan oxidizing agent. Following introduction of the oxidizing agent intothe plastic sheet, the sheet is treated in any of the developingsolutions heretofore mentioned. A water solution of from 2% to 5%concentration of any of the materials just mentioned, is suitable foruse in carrying out. these practices,

A gelatin wash-01f relief which carries a positive reproduction of aphotographic negative image may be prepared in accordance withconventional practices. For example, such a washoif relief may be formedin the manner set forth in Patent No. 2,315,373, issued March 30, 1943,

to Edwin H. Land, for Process for forming lightpolarizing images.Briefly, the relief is made by printing through a photographic negative'onto suitable wash-Joli? relief film. In this regard, the rinting stepillustrated in connection with Fig. 1 for forming an imagecomprising anoxidizing agent in light-sensitive plastic sheet is similar to thatemployed for exposing wash-01f relief film if wash-off relief film isconsidered as substituted for the sheet l0.

Following development of the wash-01f relief,

1 which is carried out by standard procedure, the

relief is saturated in a water solution of the desired oxidizing agentor agents. Usually, one or two minutes will be suflicient to suitablysaturate the wash-01f relief.

The developed wash-off relief, which is illustrated in Fig. 2 by thereference numeral 40, followingits saturation with the oxidizing agentis placed in face to face contact with the surface of the plastic sheetin which the dichroic image is to be formed whereby to print such imageon the sheet. The relief and the plastic sheet are pressed into contact,a convenient method for carrying out such a step being illustrated inFig. 2 wherein the developed and saturated relief 40 is shown superposedwith the relief side thereof upon the oriented surface of 'a sheet ofpolyvinyl alcohol 42. Fig. 2 as a matter of convenience shows the twoelements 40 and 42 supported on a flat surface such as the surface 44and any device such as a rubber roller 45 or squeegee pressing the twosheets evenly together. The relief and the plastic sheet should beallowed to remain in contact for several minutes during which time theoxidizing agent will transfer from the relief to the plastic sheeti'n-well-understood manner.

Since the amount of the oxidizing agent held by the relief variesinaccordance with the thickness of the gelatin layer at each particularpoint,

l l the oxidizing agent:will?be difierentially imbibed into-theplastiosheet proportifinally -totheness of *the -='reli'ef ;"and ibecausetheL-rvariatio the thickness of the relif corresponds -to the variationin density of theprigiiial photographic negative image sfrom which tl'ie-='relief waspro duced; a positive: reproduction of: the' negative imagef in :te'rm's 0i the oxidizing agent -willi' be formed inthe sheet 42.

Having formedan image of 'an oisidizinggagent,

dizahle material, for examplesinamacid solutionof an iodide or mixtureof iodides. Following di chroic image --fo'rmati'o'n whieh taks place'-ina mann'er'lik'ethat-alreadyset=fort heshe'etmay be washed and dried-:The efict-of='=this process is to provide a dichroicimage'inoriented'plaistic sheet similar to th'at' disclosed inconnection with Figsw2 and 3. -A --pair ot imagesproducediwthis' mannerand-forming a' le-i't -eye-ima id@ ht;

eye image mayr'be superposed ppomeachfother'to provideastereoscopicimage'which'mayi'beviewed through suitable 'viewingnevi'cesz" As stated"in com:ecti'o'irwith"the process"em": ploying a li'ghtsensitive'oxidizingxagentfthe in.-. vention may- 'be practiced to;producee uniform dichroic' light-polarizing sheet byjmbibiiig olur tionofa" relatively li ht-i-insen'sitive' oxidizing agent throughout"thesheetgandxthenitreatng the sheet, and imhib ed oxidizingwa entctocreatdiafli; chroic stain; V e K v V v 7 When formin ,liehtepolarizig-im ee bypn erential imbibitionrof anoxidizingagentiriplastic' sheet,it is also to'Joe, notdthat it'dsnot. essential torthe.pra'ctieejofgjthe; invention that. the yario'us stepsof theprocessdescribed-i'ri .connectio'nithere with. .be carried "out. .continuously';, For-example; the imbibiti'on of the oxidizingagentintheplastidsheet may take place at "an entirely di-firenttime from the remainderofnthatprocess V The, term ffoxidiz'ingiagentias used ir'i-thespecieiioation and,claimsiisiowbeluirdeiistoodsassmearie mg asingleoxidizingt naterialora mixture ofisev eral oxidizing material-s1.

It should be understood thatmanyil s of im-, ages other. thanr\photographicsnegatives may'r be reproduced/by. thisprocess;v such(for, examplesasa negative ;or= positive of @anysother:partially-alight: transmitting,.-type orian Lopaque obi'iect ofiwhioh itis. desired ,to make-.asi-lhouettei Therinycntion is particularly.adapted, vlfornexampler,to theireproductio xofi'stencils drawings;E'halfetonesa-Pr-inted mattersand the. like; Negative l Zin Fig:- 1-:isloo'uld be considered,therefore,asrepresentativeefany such similarmedium,whichmvill difierentiallwab sorb .the light incident onstl'iersensitized sheet andthe'reproductions therefromrmadeeby the sprocess ofthis inventiona will 1 be positivei repmduotions; that is tosay; theslight values-theremi-will barreversed withre spe'ct to said negative Ashas been. previously stated the ioregoingideascription'ismerelyjilustrative, .v onemt more ops verative'rfor'ms.pfxth'e; processes iofethe'present in= vention; It must:again7be'emphasizedithatseach 12 degree thereofain. one iorsmore steps;cambe compensated'for in the other steps:

Siiice'certain changes :mayhe made in carrying out the above-processwithout departing iromthe scope; of theiinventiomit is? intended thatall matter contaiiied-sin :th'eeabove description shall beinterpreted-as illustrative' and not in" a limiting sense.

Whatisclaimed .is:;;

1.: The.:processwhichccompriscs treating a carrier sheet-withsadichromatesolution.to'render it light-sensitive;exposing :saidsensitized sheet to lightirthrou-gh; :a; differentially;light-transmitting medium; removing the unexposed dichromate therefrom;and :treating: saidjsheet with an acid solutionaofuaniodiderswhichreacts with; said dichromate to 'giveaaflichroic image asan-oxidation product;

2.?3I'he process whichpomprisestreating a sheetfsaim'olecularlyroriented' transparent,- hydrophilic;linearpolymericplastic with a dichromate solution to: render; it light-sensitive,exposing said senSitized-:,Sheetf tO light throughi-ia differentiallylightl-transmittingtmedium; removing :the unex-- posed Lidichmmateetherefrom, and. treating 1 said sheet' withi anzacidic" iodidesolutionfwhich reacts with; the di'chromate ;,ther.ein. to produce -adichroic stain:

3;;The aiprocess which comprises i imbibing a sheetof.arsubstantiallyloriented, transparent, hydrophilicplinearpolymeric;plastic -in'-a dichromate solutionrforriiastimezsuch ithat only arelatively thimlayer onithe'esurface-of said-sheet is penetrated:bysaidsclution, exposing said sheet to light through: a differentially-light-transmitting: me-

dium; removinglthe unexposed dichromate therefrom, and then developingthe exposed, dichromate f in :an acidic iiodide solution which. reactswith the dichromate to producea dichroie stain in the exposedareasofsaidsheet;

4:: The process-which comprises treating a sheetOfJI-"PI'EdQtBIIHi-DBGIY molecularly oriented polyvi-nyl'alcohol with'adichromate solution to render it=v light-sensitive:exposing:saidsensitized sheet toirlightithi'oughraa differentiallylight-transmitting 1 medium,.: removing the unexposeddichromateytherefrom-,.and treating said sheet with an iodiderinz-anzacid: medium whereby the -iodide is oxidized byjthe,-exp.osedzdichromate to iodine to form an image-tinziodine in;said'zcarrier'.

5: :The process which comprises treating-a sheet of i'predetermined1y.-:mole'cularly oriented po1yvinyl alcohol with-adichromate solution torender it 1light-sensitive, exposing said sensitized sheetto?rlightathroughi :a' differentially lighttransmittirig.:medium;removing the unexposed dichromate therefrom;-andthemtreating-said sheet with an acidic iodidesisolution .WhiChfreacts :with the exposed dichitomate therein toproducea dichroicstainwinttheiexposed areas thereof;

'6:2-Thaprocesstwhich:comprises treating a sheet ofripredeterminedly"-mo1ecularly oriented polyvinyl:alcolmlyvitl'i a .d-ichromate solution to render itslightesensitivee'exposing said:fsensitized.sheet to *lighttthroughzta:"difierentially light transmittingmediumgrremoving the unexposed dichromate therefrom; andz-then: treatingsaid I sheet with *a solution: containingzan: acid and at least: one-waten-soluble iodide salt ada'ptedito react with saidexposed'dichromate 'to produce a dichroic stain initherexposedrareas ofsaid sheet.

7. Thesprocess:which' comprises treating a sheet ofipredeterminedly-rmolecu1a'rly oriented polyvinyralcohol with a -dichromate. solution torender it light sen'sitivei-exposing said-sensitizedsheet to lightthrough a differentially light-transmitting medium, removing theunexposed dichromate therefrom, and then treating said sheet with asolution containing an acid and at least one water-soluble iodide saltadapted to react with said exposed dichromate to produce a dichroicstain in the exposed areas of said sheet, said solution containing alsoa relatively weak alkali in sufiicient quantity to maintainsubstantially constant the pH value thereof.

8. The process which comprises treating a sheet of predeterminedlymolecularly oriented polyvinyl alcohol with a dichromate solution torender it light-sensitive, exposing said sensitized sheet to lightthrough a difierentially light-transmitting medium, removing theunexposed dichromate therefrom, and then treating said sheet with asolution containing an acid and at least one water-soluble iodide saltadapted to react with said exposed dichromate to produce a dichroicstain in the exposed areas of said sheet, said solution containing alsoa reducing agent.

9. The process which comprises treating a sheet of predeterminedlymolecularly oriented polyvinyl alcohol with a dichromate solution torender it light-sensitive, exposing said sensitized sheet to lightthrough a differentially light-transmitting medium, removing theunexposed dichromate therefrom, and then treating said sheet with anacidic iodide solution which reacts with the exposed dichromate thereinto produce a dichroic stain in the exposed areas thereof, said solutionalso containing an agent which is adapted to slow the penetration intothe sheet of said solution whereby to restrain softening of the sheet,said agent being substantially unreactive with the ingredients of saidsolution and said dichromate and also being a nonsolvent of polyvinylalcohol.

EDWIN H. LAND. JOSEPH MAHLER. MARK HYMAN, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 175,009 Banks Mar. 21, 1876217,404 Quartley July 8, 1879 1,214,940 Miller Feb. 6, 1917 2,185,018Sauer Dec. 26, 1939 2,204,604 Land June 18, 1940 2,409,923 Barnes Oct.22, 1946 2,409,959 Ryan Oct. 22, 1946 FOREIGN PATENTS Number CountryDate 526,780 Great Britain Sept. 25, 1940 OTHER REFERENCES Mellor, AComprehensive Treatise on Inorganic and Theoretical Chemistry, LongmansGreen and Co., New York, 1922, vol. II, page 608 cited. (Copy in Div.59.)

Babor, "General Chemistry, Crowell Co., New York, 1929, page 798 cited.

